This invention relates to hydraulic control systems for tractors.
As is well known, tractors include position control means and draft control means, either one of which is adapted to be operated in accordance with the type of the working implement mounted on the tractor. The position control means serves to maintain the implement at a constant working level in spite of differences in soil conditions, whilst the draft control means gives a constant draft force, permitting the implement to work at a constant depth in the earth even when the earth geometry is irregular in the same type of soil, so as to maintain draft resistance at a constant level despite the irregularities in the soil surface. Thus the draft control means affords a well-controlled draft force to the tractor to enable the working implement to operate without stalling of the engine and free of like troubles even in the presence of obstacles such as stones.
Accordingly, the position control means is useful to working implements which operate out of engagement with the earth or which encounter low draft resistance, such as cultivators, fertilizer applicators, seeders, potato planters, etc. On the other hand, the draft control means is advantageous to plows or like implements which require a high draft power.
However, the foregoing characteristics of the draft control means are disadvantageous where accurate agricultural operation is essential. In the presence of stones, stubs or like obstacles or differences in the consistency of soil, the tractor may possibly skip thereover, leaving the soil partially uncultivated, or in extreme cases, the working implement will undergo a pitching motion to form an undulating soil surface, producing an adverse effect on the growth of plant. This is attributable to the inherent construction of the control valve in the hydraulic system for raising and lowering the working implement as will be described below with reference to FIG. 1.
FIG. 1 shows a valve case 1 and a control valve 2 including a plunger 2a fitted in the valve case 1 as slidably inserted in a sleeve 3. The approximate midportion of the plunger 2a is constricted to provide a spool 4. The valve case 1 further includes a discharge valve 5, a check valve 6, a pump port 7, a cylinder port 8 and a tank port 9. Hydraulic press oil is forced out from an unillustrated hydraulic pump and is introduced into the valve case 1 through the pump port 7.
When the lift arms are to be raised, the control valve plunger 2a is brought to the position of FIG. 1 (I), with the discharge valve 5 closed and the check valve 6 opened, so that oil from the pump port 7 flows through the cylinder port 8 to exert back pressure on the piston of the hydraulic system for raising and lowering the implement. When the lift arms are to be brought to a halt, the control valve plunger 2a is positioned as illustrated in FIG. 1 (II) to open the discharge valve 5, through which the oil from the pump port 7 is returned to a tank by way of the tank port 9. More specifically stated, when the lift arms are to be raised, the plunger 2a is closed by the cooperation of a plunger portion 2b and the edge portion 3a of the sleeve 3. The oil forced into the pump port 7 flows to the discharge valve 5, applying back pressure to the discharge valve 5 to close the valve 5, whereby the return flow of oil to the tank port 9 is interrupted. Subsequently, the pressure of inflowing oil, when exceeding a given level, opens the check valve 6 and applies back pressure to the hydraulic system for the implement. When the lift arms are to be brought to a stop, the oil imparting back pressure to the discharge valve 5 is released through the clearance e between the edge 3a of the sleeve 3 and the edge 4a of the spool 4 to open the discharge valve 5. Oil therefore flows back. Since the edge portion 3a of the sleeve 3 is formed by boring, it is very difficult to accurately dimension the clearance e and, even if accurate working is possible, difficulties are encountered in making the clearance e smaller than 0.5 mm. The result achieved when the clearance e is 0.5 mm corresponds to about 5.degree. in terms of the angle of pivotal movement of the lift arm.
Consequently, insofar as the draft control function of tractors is afforded by a control valve of the type described, the lift arms operate with a time delay in response to the displacement of a spring or the like which senses the draft force. Thus the response of the lift arm, which should ideally respond as represented by the solid line 1 in FIG. 2, actually takes place stepwise as indicated therein by the dot line II. (In FIG. 2 the displacement of spring is plotted as abscissa vs. the amount of responsive movement of the lift arms as ordinate.)
If the relationship between the displacement of spring and the amount of response of the lift arms varies stepwise, objections will result. For example, if the tractor encounters an especially heavy soil, the load on the rear wheels increases, leading to an increase in the draft torque of the rear wheels, and the tractor becomes liable to swerve off. At the same time, the draft control means functions to relieve the working implement of the load with delayed timing. As a result, the tractor body skips or bounces, giving an irregular undulating soil surface.